Q+A with Stew MacLehose and Dan Kolbert on the performance of a well-insulated roof

When we visited Stew MacLehose’s house in 2011, it was among the first LEED Platinum homes in Maine. Designed by Kaplan Thompson Architects and built by Dan Kolbert of Kolbert Building, it now includes a recently added attic studio (see page 88). The home’s tight, double-thick wall construction, triple-glazed windows, and solar array set the standard for Kaplan Thompson’s BrightBuilt Home modular designs, and after six years it’s still a prime example of the benefits of a tight, well-insulated roof.

Q. WHAT’S UNIQUE ABOUT THE CONSTRUCTION OF THE ROOF?

KOLBERT: With this house, we developed an insulation system we’ve used since on both new and old houses. We sized the rafters for what we needed structurally (twoby- eights) and added plywood nailed to a thin strip of two-by material to create a 4-inch space below the rafters. This helped us get 14 inches of insulation (dense-packed cellulose) with an R-value of over R-50. The insulation in the 4-inch space creates a “thermal break” so that no wood runs continuously from the interior to the cold underside of the roof. This significantly reduces heat loss and condensation.

Q. WHAT ARE THE BENEFITS OF THIS KIND OF CONSTRUCTION?

KOLBERT: Besides the low cost for heating and a very quiet house, a well-insulated and air-sealed roof also minimizes the stack effect, where hot air rising through the upper floors draws in cold air from the lower floors. Even though the only heat source in the house comes from radiant tubes running under the first-floor slab, the third-floor attic is typically within five degrees of the first-floor temperature. With the redesign to use the attic for a studio, we only needed to add a small amount of supplemental heat.

Q. WHAT ARE THE ANNUAL UTILITY COSTS?

MACLEHOSE: Average annual costs for the approximately 2,200-square-foot home are $780 for electricity (supplemented by a 5-kilowatt solar array) and $855 for propane, for the gas range and fireplace. The radiant floor heating and domestic hot water run off an 88-tube solar thermal collector, which is supplemented by an on-demand electric hot water heater.